1. Field of the Invention
The present invention relates to a switching power supply circuit.
2. Description of the Related Art
One type of switching power supply is a pulse width modulation (PWM) controlled type in which the output voltage is adjusted by control of the pulse width (e.g., see Japanese Patent Laid-Open No. 61-22760 and Japanese Patent Laid-Open No. 2007-020391). There are also switching power supplies that have reduced noise and improvements in terms of loss through the use of current resonance. FIG. 9 is a typical circuit configuration diagram of a power supply circuit that uses current resonance in addition to pulse width control (referred to hereinafter as a current resonance-type PWM power supply). In FIG. 9, direct current (DC) power sources 1 and 2 are ordinarily configured so as to rectify and smooth a commercial alternating current (AC) power source. The DC power sources are connected to switching elements 3 and 4 that alternately switch ON/OFF so as to alternately allow positive and negative currents to flow to the primary coil of a transformer 7. A pulse width control unit 15 supplies a pulse signal for controlling the ON/OFF switching of the switching elements 3 and 4.
The pulse width control unit 15 is realized by a dedicated IC, for example, and has circuits for performing oscillation, reference voltage generation, error amplification, and pulse width control. The current that flows to the transformer 7 returns to the DC power sources 1 and 2 via an inductor 6 and a capacitor 5. The capacitance of the capacitor 5 is selected such that the series resonance frequency, which is determined by the combination with the inductor 6, is slightly higher than the pulse frequency. Here, the inductor 6 is an independent element or a leakage inductor of the primary coil of the transformer 7. The AC output of an amplified output coil 9, which is induced by the current flowing to the primary coil 8 of the transformer 7, is rectified and smoothed by a bridge diode 11 and a smoothing capacitor 12 so as to obtain a DC voltage. The DC output voltage is applied to an external load 14, which is the power supply destination. Note that the external load 14 in FIG. 9 equivalently represents the input impedance of an arbitrary load circuit (e.g., an amplifier).
Also, the DC output is connected to error detection resistors 13 that divide the voltage that is to be applied to the load, and send the divided voltage to the pulse width control unit 15. An error amplifier in the pulse width control unit amplifies errors between the divided voltage and a reference voltage, and the pulse width control circuit controls the pulse width that is output, based on the errors. Specifically, the pulse width control circuit operates so as to reduce the pulse width if the divided voltage is higher than the reference voltage, and increase the pulse width if the divided voltage is lower than the reference voltage. In this way, the DC voltage applied to the load resistor 14 is kept constant. Specifically, if the resistance value of the external load 14 has changed to a low value, the load current increases and the DC output voltage decreases, and therefore feedback is applied such that the pulse width increases (such that the voltage increases), and thus the output voltage is controlled so as to be constant.
When a high-output audio amplifier is envisioned as the external load to which power is supplied, a difference of 7.5 times, for example, exists between the DC voltage for amplified output (for the amplification operation) (e.g., 90 V) and the DC voltage for audio signal processing (e.g., 12 V). Therefore from the viewpoint of efficiency, a secondary amplified output coil 9 and a signal system coil (not shown) are provided as separate secondary coils, as shown in FIG. 10. Also, since the pulse width control unit 15 operates with a low voltage (e.g., 12 V), it is conceivable to similarly provide a control coil 10 for drawing a control DC voltage for the pulse width control unit 15. By inputting the AC output of the control coil 10 to the pulse width control unit 15 as the control DC voltage via a rectifying/smoothing circuit 17, it is possible to perform control such that the output voltage of the rectifying/smoothing circuit 17 is constant. Note that in order to prevent power supply switching noise from having a negative influence on the audio signal processing system in this case, it is envisioned that the control coil 10 is provided as a coil that is separate from the signal system coil (not shown).
However, in the circuit shown in FIG. 10, there is the possibility that the DC voltage for audio signal processing and the DC voltage in the amplified output stage are not controlled so as to be constant voltages. Specifically, if the audio input signal that is input to the amplifier is low, and the amplified output is low, the load impedance value will be a high value. For this reason, the sink current will be low, and the speed of response to voltage drops due to discharge in the smoothing capacitor 12 will be slow, thus resulting in the possibility of the problem that constant voltage control is not performed properly, and the voltage is relatively high.
Also, if the audio input signal that is input to the amplifier is high, and the amplified output is high, the influence of the leakage inductor will be relatively high due to a limit on the core size of the transformer (limit on number of turns). For example, the number of turns of the control coil is four, and the number of turns of the amplified output coil is 30. The control DC voltage that is subjected to constant voltage control is obtained by the rectification and smoothing of the AC voltage that appears in the leakage inductor and the inductor of the original coil portion, and the AC voltage of the original coil portion is relatively low. In other words, the voltage in the amplified output stage is relatively low since the AC voltage of the original coil portion is reflected in the AC voltage of the amplified output coil 9. The influence of this is remarkable particularly in the case of high output. Furthermore, in the case of high output, the current that flows to the resonance circuit is high, and changes in the pulse width are suppressed by the influence of the Q value of the resonance circuit. For this reason, it is also conceivable that precision will decrease in the constant voltage control performed on the control coil 10, and that the voltage in the amplified output stage will decrease. In other words, there is the possibility of the problem that rated output is not obtained in the case of a high load.